Preliminary evidence suggests that muscle tissues incubated in plasma rather than Ringer's solution behave differently in regards to ion distribution and exchange, which may be due to a regulatory effect of some plasma fraction on membrane transport properties. This may in part be the reason for the differences observed between skeletal and cardiac muscles studied in situ and in vitro in regards to membrane transport properties. Therefore, the object of this proposal is to reevaluate classical in vitro measurements of membrane potential, Em, cellular ion concentration and ion activity, a i over i, and cellular ion exchange rates for Cl and later for other univalent cations (K and Na), using mammalian skeletal muscle incubated in both plasma and in Ringer's solution. An appropriate mammalian perfused hindlimb model will be used as an in situ baseline of comparison for the in vitro observations. This proposed project should permit a better understanding of the mechanisms which operate to control membrane function in situ. The long-range goal is to develop an appropriate in situ or in vitro model which more closely approximates the physiological steady state condition. This model can then be used as an important additional physiological preparation to examine the effects of various chemical and physical perturbations on ion distribution and exchange in muscle.